﻿COOKE 
  : 
  FORM 
  VARIATION 
  IN 
  SIPHONARIA 
  ALGESIB^S. 
  

  

  355 
  

  

  far 
  from 
  double, 
  while 
  the 
  ratio 
  of 
  height 
  to 
  length 
  is 
  in 
  the 
  one 
  case 
  

   •20 
  cm., 
  in 
  the 
  latter 
  -35 
  cm., 
  or 
  75 
  per 
  cent 
  higher. 
  

  

  L 
  M 
  N 
  

  

  The 
  conclusion 
  can 
  scarcely 
  be 
  resisted 
  that 
  the 
  flattening 
  of 
  the 
  

   shell 
  on 
  the 
  reefs 
  towards 
  low-water 
  mark 
  is 
  an 
  advantage 
  to 
  the 
  

   organism, 
  enabling 
  it 
  to 
  avoid 
  the 
  strong 
  iinpact 
  of 
  the 
  waves, 
  whose 
  

   power 
  to 
  detach 
  must 
  decrease 
  in 
  direct 
  ratio 
  with 
  the 
  smaller 
  amount 
  of 
  

   resistance 
  they 
  encounter. 
  Given 
  an 
  equal 
  capacity 
  for 
  ' 
  holding 
  on 
  ', 
  

   an 
  elevated 
  shell 
  would 
  have 
  more 
  chance 
  of 
  being 
  knocked 
  off 
  by 
  the 
  

   force 
  of 
  breaking 
  waves 
  than 
  a 
  comparatively 
  flat 
  shell 
  which 
  allowed 
  

   the 
  water 
  to 
  run 
  over 
  it. 
  On 
  the 
  other 
  hand, 
  in 
  a 
  station 
  where 
  the 
  

   impact 
  of 
  a 
  wave 
  was 
  rarely 
  felt, 
  and 
  where 
  there 
  was 
  comparatively 
  

   little 
  need 
  to 
  provide 
  against 
  sudden 
  shocks, 
  an 
  elevated 
  shell 
  would 
  

   suffer 
  no 
  disadvantage, 
  and 
  would 
  therefore 
  tend 
  to 
  become 
  the 
  rule. 
  

   The 
  facts 
  as 
  observed 
  are 
  in 
  direct 
  accordance 
  with 
  this 
  theory. 
  

  

  In 
  a 
  very 
  interesting 
  account 
  of 
  the 
  common 
  limpet 
  Mr. 
  E. 
  S. 
  

   Russell 
  ^ 
  arrives 
  at 
  very 
  similar 
  conclusions. 
  He 
  observes 
  : 
  "To 
  sum 
  

   up, 
  adult 
  exposed 
  shells 
  of 
  Patella 
  are 
  lower-spired, 
  narrower, 
  thicker, 
  

   and 
  more 
  irregular 
  in 
  outline 
  than 
  sheltered 
  shells 
  ; 
  and 
  these 
  

   differences 
  are 
  probably 
  due 
  to 
  the 
  difference 
  of 
  friction 
  to 
  which 
  the 
  

   two 
  kinds 
  of 
  shells 
  are 
  exposed 
  from 
  waves 
  and 
  tide." 
  

  

  The 
  Rev. 
  Canon 
  jSTorman- 
  also 
  remarks: 
  " 
  It 
  will 
  he 
  found 
  to 
  be 
  

   a 
  general 
  rule 
  with 
  regard 
  to 
  the 
  limpet 
  that 
  the 
  nearer 
  high-water 
  

   mark 
  the 
  shells 
  are 
  taken, 
  the 
  higher-spired, 
  more 
  strongly 
  ribbed, 
  and 
  

   smaller 
  it 
  will 
  be 
  ; 
  and 
  that 
  the 
  lower 
  down 
  it 
  lives, 
  the 
  flatter, 
  less 
  

   ribbed, 
  and 
  larger 
  it 
  becomes." 
  Mr. 
  Russell, 
  commenting 
  upon 
  this 
  

   statement, 
  says 
  : 
  " 
  While 
  Tuy 
  observations 
  entirely 
  corroborate 
  the 
  

   truth 
  of 
  the 
  statement 
  that 
  hujh-ioater 
  limpets 
  are 
  typically 
  high- 
  

   spired, 
  yet 
  I 
  find, 
  for 
  one 
  area 
  at 
  least, 
  that 
  the 
  proportion 
  of 
  large 
  

   shells 
  is 
  greater 
  at 
  high-water 
  than 
  at 
  low-water." 
  

  

  ^ 
  Proc. 
  Zool. 
  Soc, 
  pp. 
  856-70, 
  1907. 
  

  

  The 
  Zoologist, 
  p. 
  7202, 
  1860. 
  

  

  